Lubricating oil composition for internal combustion engine

ABSTRACT

A lubricating oil composition for an internal combustion engine, which includes a base oil, (A) at least one member selected from the group consisting of disulfide compounds represented by the general formula (I): 
       R 1 OOC-A 1 -S—S-A 2 -COOR 2   (I) 
     and disulfide compounds represented by the general formula (II): 
       R 7 OOC—CR 9 R 10 —CR 11 (COOR 8 )—S—S—CR 16 (COOR 13 )—CR 14 R 15 —COOR 12   (II), 
     and (B) at least one member selected from the group consisting of alkali metal-based detergents and alkaline earth metal-based detergents, the component (B) being present in an amount of 10 to 2,000 ppm by mass in terms of metal content. 
     The lubricating oil composition has a low ash content, a low phosphorous content and a low sulfur content. But, Nonetheless, it can maintain wear resistance, has excellent heat resistance, can extend the lubricating oil change interval and has a long service life.

TECHNICAL FIELD

The present invention relates to a lubricating oil composition for aninternal combustion, particularly to a lubricating oil compositionsuitable for use in an internal combustion engine such as a gasolineengine, a diesel engine or a gas engine. More specifically, the presentinvention is directed to a lubricating oil composition which, despiteits low ash content, low phosphorus content and low sulfur content, canmaintain excellent wear resistance, exhibits excellent heat resistance,can extend the lubricating oil change interval and has a long servicelife.

BACKGROUND ART

A lubricating oil for an internal combustion engine mainly functions tolubricate various sliding portions such as piston rings, cylinderliners, crankshafts, connecting rods and a valve-operating mechanismincluding a cam and a valve-lifter. In addition, the lubricating oilfunctions to cool the inside of the engine and to disperse sludge anduncombusted residues of the fuel.

Thus, lubricating oils for internal combustion engines are required tohave various properties. Further, in recent years, lubricating oils arerequired to exhibit high performance, since internal combustion engineshave been improved in their performance and output and since theoperation conditions thereof have become more severe. In order tosatisfy the required performance, various additives such as an wearinhibitor, a metal detergent, an ashless dispersant and an antioxidantare compounded in the lubricating oils.

As a part of the improvement of the service life of lubricating oils,studies have been made on the development of more effective antioxidantsand combination thereof. Among others, as an extreme-pressure additive,a zinc alkyldithiophosphate (ZnDTP) has been used a lot. The ZnDTP doesnot only function as an antioxidant but also has a significant effect toprevent wear and corrosion and, therefore, has been widely used inengine oils, in particular.

Recent tightening of automobile exhaust gas emission regulationsrequires mounting of an exhaust gas cleaning device for gasolineengines. In order to prevent poisoning of a three-way catalyst,lead-free gasoline and low-phosphorus engine oils begin to be used. Forthis reason, the use of ZnDTP begins to be subjected to restrictions.Thus, it is essential to develop an extreme-pressure antiwear agent,which is free of phosphorus.

In diesel engines, since fine particulate matters contained incombustion gases exhausted from the diesel cars cause a problem, it isobliged to mount a diesel particulate filter (hereinafter referred to as“DPFF” for brevity) on the diesel cars. When ZnDTP is used as anextreme-pressure additive in the same manner as above, the zinccomponent contained in ZnDTP deposits on DPF may cause clogging of thefilter. In this circumstance, there is a demand for the development of asubstitute for ZnDTP.

From the viewpoint of catalyst poisoning, it is desired to reduce aphosphorus component contained in ZnDTP.

Patent Document 1 discloses a lubricating oil composition which containsa metal salt or amine salt of a thiophosphate or of a phosphate andwhich is alleged to have a reduced sulfur content and excellentproperties to maintain its base number. Though the composition canreduce the sulfur content as a replacement for ZnDTP, reduction of aphosphorus component and a zinc (ash) component is not achieved. Thus,problems of catalyst poisoning and clogging of DPF remain unsolved.Namely, the composition is still unsatisfactory as a lubricating oilcomposition.

As described above, it is difficult to decrease a zinc component, aphosphorus component and a sulfur component without reducing the wearresistance and antioxidizing properties of an extreme-pressure additive.Yet, reduction of such components is desired in order to minimizeadverse influence upon post-treatment systems for automobiles, such asan exhaust gas treatment device and DPF, as much as possible.

It is conventionally well known that a combination of a sulfur-basedantioxidant with a phenol-type antioxidant gives a synergetic effect. Asthe sulfur-based antioxidant, mainly known is one which has amonosulfide structure. Such a sulfur-based antioxidant, however, has aproblem because it causes an increase of the acid value by hydrolysis.Sulfur-based compounds having a polysulfide (tri- or higher polysulfide)structure has a problem that it is highly corrosive to non-iron metals.

[Patent Document 1] Publication No. 2002-924721

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

Under the above-mentioned circumstance, an object of the presentinvention is to provide a lubricating oil composition which, despite itslow ash content, low phosphorus content and low sulfur content, canmaintain excellent wear resistance, exhibits excellent heat resistance,can extend the lubricating oil change interval and has a long servicelife.

Means for Solving the Problem

The present inventors have made an earnest study with a view towarddeveloping a lubricating oil composition for internal combustion engineshaving the above-described suitable properties and have found that acombination of a disulfide compound having a specific structure with aspecific amount of a detergent can accomplish the above object. Thepresent invention has been completed based on the above finding.

That is, the present invention provides:

(1) A lubricating oil composition for an internal combustion engine,including a base oil; (A) at least one member selected from the groupconsisting of disulfide compounds represented by the general formula(I):

R¹OOC-A¹-S—S-A²-COOR²  (I)

(wherein R¹ and R² each independently represent a C₁ to C₃₀ hydrocarbylgroup which may contain an oxygen atom, a sulfur atom or a nitrogenatom, and A¹ and A² each independently represent a group expressed byCR³R⁴ CR³R⁴—CR⁵R⁶ where R³ to R⁶ each independently represent a hydrogenatom or a C₁ to C₂₀ hydrocarbyl group), and disulfide compoundsrepresented by the general formula (II):

R⁷OOC—CR⁹R¹⁰—CR¹¹(COOR)—S—S—CR¹⁶(COOR¹³)—CR¹⁴R¹⁵—COOR¹²  (II)

(wherein R⁷, R⁸, R¹² and R¹³ each independently represent a C₁ to C₃₀hydrocarbyl group which may contain an oxygen atom, a sulfur atom or anitrogen atom and R⁹ to R¹¹ and R¹⁴ to R¹⁴ each independently representa hydrogen atom or C₁ to C₅ hydrocarbyl group), and (B) at least onemember selected from the group consisting of alkali metal-baseddetergents and alkaline earth metal-based detergents, the component (B)being present in an amount of 10 to 2,000 ppm by mass in terms of metalcontent;(2) The lubricating oil composition for an internal combustion engine asdefined above (1), wherein the component (A) being at least one memberselected from the disulfide compounds is present in an amount of 0.01 to0.50% by mass in terms of sulfur content;(3) The lubricating oil composition for an internal combustion engine asdefined in above (1) or (2), wherein the alkali metal-based or alkalineearth metal-based detergent of the component (B) is a salicylate and/ora sulfonate; and(4) The lubricating oil composition for an internal combustion engine asdefined in any one of above (1) to (3), wherein the metal of thecomponent (B) is Ca or Mg.

Effect of the Invention

According to the present invention, a lubricating oil composition forinternal combustion engines, which, despite its low ash content, lowphosphorus content and low sulfur content, can maintain excellent wearresistance, exhibits excellent heat resistance, can extend thelubricating oil change interval and has long service life can beprovided.

BEST MODE FOR CARRYING OUT THE INVENTION

A lubricating oil composition for internal combustion engines of thepresent invention (hereinafter occasionally referred to simply as“lubricating oil composition”) contains a base oil which may be amineral oil or a synthetic oil. Various types of mineral oils andsynthetic oils are available, and a suitable oil can be suitablyselected in accordance with the intended use. Examples of the mineraloil include paraffinic mineral oils, naphthenic mineral oils andintermediate mineral oils. Specific examples of the mineral oil includelight neutral oil, intermediate neutral oil, heavy neutral oil andbright stock. which are obtained by solvent refining or purification byhydrorefining.

Examples of the synthetic oil include poly-α-olefins, α-olefincopolymers, polybutene, alkylbenzenes, polyol esters, esters of dibasicacids, polyhydric alcohol esters, polyoxyalkylene glycols,polyoxyalkylene glycol esters, polyoxyalkylene glycol ethers andcycloalkane compounds. These lube base oils may be used singly or incombination of two or more thereof. A combination of a mineral oil witha synthetic oil may be also used.

The compound of the general formula (I) used as the component (A) of thelubricating oil of the present invention has the following structure:

R¹OOC-A¹-S—S-A²-COOR²  (I)

In the general formula (I), R¹ and R² each independently represent a C₁to C₃₀ hydrocarbyl group (which may contain an oxygen atom, a sulfuratom or a nitrogen atom), preferably a C₁ to C₂₀, more preferably C₂ toC₁₈, particularly preferably C₃ to C₁₈ hydrocarbyl group. Thehydrocarbyl group may be straight chained, branched or cyclic. R¹ and R²may be the same or different but are preferably the same for reasons ofeasy production.

Next, A¹ and A² each independently represent a group expressed by CR³R⁴or CR³R⁴—CR⁵R⁶ where R³ to R⁶ each independently represent a hydrogenatom or a C₁ to C₂₀ hydrocarbyl group. The hydrocarbyl group ispreferably a C₁ to C₁₂, more preferably C₁ to C₈ hydrocarbyl group. A¹and A² may be the same or different but are preferably the same forreasons of easy production.

In the lubricating oil composition of the present invention, it ispreferred that the content of tri- or higher poly-sulfides (namely—S_(x)— where x is 3 or more; namely —S—S—S— or higher sulfide bonds) inthe general formula (I) be 30% by mass or less based on a total amountof the disulfide compound. When the polysulfide content is 30% by massor less, corrosiveness to non-iron metals may be suppressed. The contentof tri- or higher poly-sulfide is more preferably 10% by mass or less,particularly preferably 5% by mass or less.

Thus, it is important that the disulfide compound represented by theabove general formula (I) should be produced by a method in which theamount of tri- or higher poly-sulfide compounds by-produced is withinthe above range. In the present invention, the disulfide compound may bepreferably produced by the following method.

Namely, an ester of a mercaptoalkanecarboxylic acid represented by thefollowing general formula (III) and/or general formula (IV) as a rawmaterial is subjected to oxidative coupling:

R¹OOC-A¹-SH  (III)

R²OOC-A²-SH  (IV)

(wherein R¹, R², A¹ and A² are as defined above). With the aboveproduction method, tri- or higher poly-sulfide compounds aresubstantially not by-produced. Specifically produced areR¹OOC-A¹-S—S-A²-COOR², R¹OOC-A¹-S—S-A¹-COOR¹ and R²OOC-A²-S—S-A²-COOR².

As an oxidizing agent for oxidizing an α-mercaptocarboxylic acid esterfor the production of the corresponding disulfide, there may be used anoxidizing agent used for producing a disulfide from mercaptan. Examplesof such an oxidizing agent include oxygen, hydrogen peroxide, a halogen(such as iodine or bromine), a hypohalous acid and a salt thereof, asulfoxide (such as dimethyl sulfoxide or diisopropyl sulfoxide) andmanganese (IV) oxide. Among these oxidizing agents, oxygen, hydrogenperoxide and dimethyl sulfoxide are preferred because they areinexpensive and make it easy to produce a disulfide.

Specific examples of the disulfide compound represented by the generalformula (I) include bis(methoxycarbonylmethyl) disulfide,bis(ethoxycarbonylmethyl)disulfide, bis(n-propoxycarbonylmethyl)disulfide, bis(isopropoxy-carbonylmethyl) disulfide,bis(n-butoxycarbonylmethyl) disulfide,bis(n-octoxycarbonylmethyl)disulfide, bis(n-dodecyloxycarbonylmethyl)disulfide, bis(cyclopropoxycarbonylmethyl)disulfide,1,1-bis(1-methoxycarbonylethyl)disulfide,1,1-bis(1-methoxycarbonyl-n-propyl) disulfide,1,1-bis(1-methoxycarbonyl-n-butyl) disulfide,1,1-bis(1-methoxycarbonyl-n-hexyl)disulfide,1,1-bis(1-methoxycarbonyl-n-octyl)disulfide,1,1-bis(1-methoxycarbonyl-n-dodecyl)disulfide,2,2-bis(2-methoxycarbonyl-n-propyl)disulfide,α,α-bis(α-methoxycarbonylbenzyl)disulfide,1,1-bis(2-methoxycarbonylethyl)disulfide,1,1-bis(2-ethoxycarbonylethyl)disulfide,1,1-bis(2-n-propoxycarbonylethyl)disulfide,1,1-bis(2-isopropoxycarbonylethyl)disulfide,1,1-bis(2-cyclopropoxycarbonylethyl)disulfide,1,1-bis(2-methoxycarbonyl-n-propyl)disulfide,1,1-bis(2-methoxycarbonyl-n-butyl)disulfide,1,1-bis(2-methoxycarbonyl-n-hexyl) disulfide,1,1-bis(2-methoxycarbonyl-n-propyl) disulfide,2,2-bis(3-methoxycarbonyl-n-pentyl)disulfide and1,1-bis(2-methoxycarbonyl-1-phenethyl)disulfide,

The compound of the general formula (II) used as the component (A) ofthe lubricating oil composition of the present invention has thefollowing structure:

R⁷OOC—CR⁹R¹⁰—CR¹¹(OOR⁸)—S—S—CR¹⁶(COOR¹³)—CR¹⁴R¹⁵—COOR¹²  (II).

In the general formula (II), R⁷, R⁸, R¹² and R¹³ each independentlyrepresent a C₁ to C₃₀ hydrocarbyl group (which may contain an oxygenatom, a sulfur atom or a nitrogen atom), preferably a C₁ to C₂₀, morepreferably C₂ to C₁₈, particularly preferably C₃ to C₁₈ hydrocarbylgroup. The hydrocarbyl group may be straight chained, branched orcyclic. R⁷, R⁸, R¹² and R¹³ may be the same or different but arepreferably the same for reasons of easy production.

Next, R⁹ to R¹¹ and R¹⁴ to R¹⁶ each independently represent a hydrogenatom or C₁ to C₅ hydrocarbyl group. For reasons of easy materialavailability, a hydrogen atom is preferred.

The above disulfide compound is preferably produced by, for example, thefollowing two methods. In the first production method, a diester of amercaptoalkanedicarboxylic acid represented by the following generalformula (V) and/or general formula (VI) as a raw material is subjectedto oxidative coupling:

R⁷OOC—CR⁹R¹⁰—CR¹¹(COOR⁸)—SH  (V)

R¹²OOC—CR¹⁵R¹⁴—CR¹⁶(COOR¹³)—SH  (VI)

wherein R⁷ to R¹⁶ are as defined above.

Specifically produced areR⁷OOC—CR⁹R¹⁰—CR¹¹(COOR⁸)—S—S—CR¹⁶(COOR¹³)—CR¹⁴CR¹⁵—COR¹²,R⁷OOC—CR⁹R¹⁰—COR¹¹(COOR⁸)—S—S—CR¹⁶(COOR⁸)—CR¹⁰CR⁹—COOR⁷ andR¹²OOC—CR¹⁵R¹⁴—CR¹⁶(COOR¹³)—S—S—CR⁶ (COOR¹³)—CR¹⁴CR¹⁵—COOR¹².

As an oxidizing agent, there may be mentioned oxygen, hydrogen peroxide,a halogen (such as iodine or bromine), a hypohalous acid and a saltthereof, a sulfoxide (such as dimethyl sulfoxide or diisopropylsulfoxide) and manganese (IV) oxide. Among these oxidizing agents,oxygen, hydrogen peroxide and dimethyl sulfoxide are preferred becausethey are inexpensive and make it easy to produce a disulfide.

In the second production method, a mercaptoalkanedicarboxylic acidrepresented by the following general formula (VII) and/or generalformula (VIII) as a raw material is subjected to oxidative coupling:

HOOC—CR⁹R¹⁰—CR¹¹(COOH)—SH  (VII)

HOOC—CR¹⁴R¹⁵—CR¹⁶(COOH)—SH  (VIII)

wherein R⁹ to R¹¹ and R¹⁴ to R¹⁶ are as defined above. The product isthen esterified with a monohydric alcohol having a C₁ to C₃₀ hydrocarbylgroup which may contain an oxygen atom, a sulfur atom or a nitrogenatom. Specifically produced by the oxidative coupling are;

HOOC—CR⁹R¹⁰—CR¹¹(COOH)—S—S—CR¹⁶(COOH)CR¹⁵R¹⁴—COOH,

HOOC—CR⁹R¹⁰—CR¹¹(COOH)—S—S—CR¹¹(COOH)—CR¹⁰R⁹—COOH, and

HOOC—CR¹⁴R¹⁵—CR¹⁶(COOH)—S—S—CR¹⁶(COOH)—CR¹⁵R¹⁴—COOH

As an oxidizing agent, there may be used the above-described oxidizingagent.

The oxidative coupling is followed by the esterification with an alcoholrepresented by the following general formula (IX):

R¹⁷—OH  (IX)

wherein R¹⁷ is the same as the above R⁷. The esterification may becarried out by an ordinary method using an acid catalyst.

Specifically produced by the above method are:

R¹⁷OOC—CR⁹R¹⁰—CR¹¹(COOR¹⁷)—S—S—CR¹⁶(COOR¹⁷)—CR¹⁵R¹⁴—COOR¹⁷,

R¹⁷OOC—CR⁹R¹⁰—CR¹(COOR¹⁷)—S—S—CR¹¹(COOR¹⁷)—CR¹⁰R⁹—COOR¹⁷ and

R¹⁷OOC—CR¹⁴R¹⁵CR¹⁶(COOR¹⁷)—S—S—CR¹⁶(COOR¹⁷)—CR¹⁵R¹⁴—COOR¹⁷.

Specific examples of the disulfide compound represented by the generalformula (II) include tetramethyl dithiomalate, tetraethyl dithiomalate,tetra-1-propyl dithiomalate, tetra-2-propyl dithiomalate, tetra-1-butyldithiomalate, tetra-2-butyl dithiomalate, tetraisobutyl dithiomalate,tetra-1-hexyl dithiomalate, tetra-1-octyl dithiomalate,tetra-1-(2-ethyl)hexyl dithiomalate, tetra-1-(3,5,5-trimethyl)hexyldithiomalate, tetra-1-decyl dithiomalate, tetra-1-dodecyl dithiomalate,tetra-1-hexadecyl dithiomalate, tetra-1-octadecyl dithiomalate,tetrabenzyl dithiomalate, tetra-α-(methyl)benzyl dithiomalate,tetra-α,α-dimethylbenzyl dithiomalate, tetra-1-(2-methoxy)ethyldithiomalate, tetra-1-(2-ethoxy)ethyl dithiomalate,tetra-1-(2-butoxy)ethyl dithiomalate, tetra-1-(2-ethoxy)ethyldithiomalate, tetra-1-(2-butoxy)ethyl dithiomalate,tetra-1-(2-ethoxy)ethyl dithiomalate, tetra-1-(2-butoxybutoxy)ethyldithiomalate and tetra-1-(2-phenoxy)ethyl dithiomalate.

The disulfide compounds represented by the above general formulas (I)and (II) have excellent load-carrying capacity and wear resistance as asulfur-based extreme-pressure additive and is used as an additive forthe lubricating oil composition.

In the lubricating oil composition of the present invention, one of ortwo or more of the disulfide compounds represented by the above generalformula (I) may be used as the component (A). Also, one of or two ormore of the disulfide compounds represented by the above general formula(II) may be used as the component (A).

Further, a mixture of at least one of the disulfide compoundsrepresented by the above general formula (I) and at least one of thedisulfide compounds represented by the above general formula (II) may beused.

The content of the component (A), the disulfide compound, in thelubricating oil composition of the present invention may be suitablydetermined in view of the intended use and using conditions of thecomposition but is generally preferably 0.01 to 0.50% by mass, morepreferably 0.01 to 0.30% by mass, in terms of sulfur content.

It is necessary that the lubricating oil composition of the presentinvention should contain 10 to 2,000 ppm by mass, in terms of metal, ofat least one member selected from alkali metal-based detergents andalkaline earth metal-based detergents, as component (B). The content ofthe component (B) is preferably 100 to 2,000 ppm by mass, morepreferably 200 to 2,000 ppm by mass.

When the content, in terms of metal, of the component (B) is within theabove range, an increase of the ash content can be prevented withoutreducing the acid neutralization power, so that the clogging of OPF canbe prevented. Additionally, formation of deposits can be prevented, andlubricating oil change interval can be extended.

The alkali metal-based or alkaline earth metal-based detergent of thecomponent (B) (hereinafter occasionally referred to as “metal-baseddetergent” for brevity) is preferably used to improve the acidneutralization power, high-temperature detergency, wear preventingproperty, etc. The metal-based detergent is not specifically limited,i.e. any metal-based detergent commonly used for lubricating oils may beused. Specific examples of the metal-based detergent include at leastone metal-based detergent selected from alkali metal sulfonates,alkaline earth metal sulfonates, alkali metal phenates, alkaline earthmetal phenates, alkali metal salicylates and alkaline earth metalsalicylates. As the alkali metal, there may be mentioned sodium andpotassium. As the alkaline earth metal, there may be mentionedmagnesium, calcium and barium. Especially preferably used is magnesiumor calcium of an alkaline earth metal.

In the present invention, it is preferred that an alkali metal sulfonateand/or an alkaline earth metal salicylate be used for reasons ofobtaining a composition having improved base number maintainingproperty, high-temperature detergency and wear preventing property. Themetal-based detergent preferably has a total base number of 20 to 600 mgKOH/g (JIS K2501; perchloric acid method). A total base number of theabove range can ensure the ability to neutralize acidic componentsformed by oxidation, can suppress an increase of ash in the lubricatingoil and can prevent the formation of a large amount of deposits during along-term use.

The alkaline earth metal sulfonate, which is an alkaline earth metalsalt of any of various sulfonic acids, may be generally obtained bycarbonating an alkaline earth metal salt of any of various sulfonicacids. As the sulfonic acid, there may be mentioned an aromaticpetroleum sulfonic acid alkylsulfonic acid, an arylsulfonic acid and analkylarylsulfonic acid. Specific examples include dodecylbenzenesulfonicacid, dilaurylcetylbenzenesulfonic acid, paraffin wax-substitutedbenzenesulfonic acid, polyolefin-substituted benzenesulfonic acid,polyisobutylene-substituted benzenesulfonic acid and naphthalenesulfonicacid,

The alkaline earth metal salicylate, which is an alkaline earth metalsalt of an alkylsalicylic acid, may be generally produced by a method inwhich phenol is alkylated with a C₈ to C₁₈ α-olefin, the resultingproduct being then successively subjected to the Kolbe-Schmitt reactionfor introducing a carboxyl group, to double-decomposition and tocarbonation. Specific examples of the alkylsalicylic acid includedodecylsalicylic acid, dodecylmethylsalicylic acid, tetradecylsalicylicacid, hexadecylsalicylic acid, octadecylsalicylic acid anddioctylsalicylic acid.

The lubricating oil composition of the present invention may contain,depending upon its intended use, a variety of additives such as afriction modifier other than the above-mentioned, an antiwear agent, anashless dispersant, a viscosity index improver, a pour-point depressant,a rust preventive agent, a metal corrosion inhibitor, an antifoamingagent, a surfactant and an antioxidant.

As the friction modifier and antiwear agent, there may be mentioned, forexample, sulfur-based compounds such as sulfurized olefins, dialkylpolysulfides, diarylalkyl polysulfides and diaryl polysulfides;phosphorus-based compounds such as esters of phosphoric acid, esters ofthiophosphoric acid, esters of phosphorous acid, alkylhydrogenphosphites, amine salts of esters of phosphoric acid and aminesalts of esters of phosphorous acid; chlorine-based compounds such aschlorinated fats and oils, chlorinated paraffins, chlorinated fatty acidesters and chlorinated fatty acids; ester-based compounds such as estersof alkylmaleic acids and alkenylmaleic acids and esters of alkylsuccinicacids and alkenylsuccinic acids; organic acid-based compounds such asalkylmaleic acids, alkenylmaleic acids, alkylsuccinic acids andalkenylsuccinic acids; and organometallic compounds such as salts ofnaphthenic acid, zinc dithiophosphate (ZnDTP), zinc dithiocarbamate(ZnDTC), sulfurized oxymolybdenum organophosphoro dithioate (MoDTP) andsulfurized oxymolybdenum dithiocarbamate (MoDTC).

As the ashless dispersant there may be mentioned, for example,succinimides, succinimides containing boron, benzylamines, benzylaminescontaining boron, esters of succinic acid and amides of monobasic ordibasic carboxylic acids typically examples, which include fatty acidsand succinic acid.

As the viscosity index improver, there may be mentionedpolymethacrylates, dispersion type polymethacrylates, olefin-basedcopolymers such as ethylene-propylene copolymers, dispersion typeolefin-based copolymers and styrene-based copolymer such asstyrene-diene copolymers. As the pour point depressant, there may bementioned, for example, polymethacrylates.

As the rust preventive agent, there may be mentioned, for example,alkenylsuccinic acid and partial esters thereof. As the metal corrosioninhibitor, there may be mentioned, for example, benzotriazole-basedagents, benzimidazole-based agents, benzothiazole-based agents andthiadiazole-based agents. As the defoaming agent, there may bementioned, for example, dimethylpolysiloxane and polyacrylates. As thesurfactant, there may be mentioned, for example, polyoxyethylenealkylphenyl ethers.

As the antioxidant, there may be mentioned, for example, amine-basedantioxidants such as alkylated diphenylamines, phenyl-α-naphthylamineand alkylated naphthylamines, and phenol-based antioxidants such as2,6-di-t-butyl-cresol and 4,4′-methylenebis(2,6-di-t-butylphenol).

The lubricating oil composition of the present invention is, despite itslow ash content, low phosphorus content and low sulfur content,excellent in wear resistance and in heat resistance, can extend thelubricating oil change interval and has a long service life. Thus, thelubricating oil composition is mainly used as the lubricating oil forinternal engines. Further, the lubricating oil composition is used asautomobile lube oils for driving instruments such as automatictransmissions, shock absorbers and power steerings, and for gears; asmetal working oils for metal working such as cutting, grinding anddeformation processing, and as hydraulic oils, being power transmissionfluids, for transmission of power, power control and shock absorbing inhydraulic systems such as hydraulic apparatuses and instruments.

EXAMPLES

The present invention will be next described in further detail by way ofExamples but is not limited to these Examples in any way.

The analysis and evaluation of the lubricating oil compositions preparedusing the compounding formulations shown in Table 1 were determined bythe following methods.

(1) Measurement of Phosphorus Concentration

Sample is subjected to emission spectral analysis by ICP (InductivelyCoupled Plasma) analysis (apparatus: IRIS Advantage manufactured byJARRELL ASH Inc.) to determine the content (% by mass) of phosphorus inthe sample. The results of the measurement are shown in Table 1.

(2) Measurement of Sulfur Concentration

Sulfur concentration was measured in accordance with ASTM D-1552.

(3) Sulfate Ash

Sulfate ash was determined in accordance with JIS K2272, “Crude Oil andPetroleum Products; Testing Method for Ash and Sulfated Ash”.

(4) Base Number (Hydrochloric Acid Method)

Base number was measured by the potentiometric titration method (basenumber; hydrochloric acid method) in accordance with JIS K2501“Petroleum products and lubricants; Testing Method for NeutralizationNumber”.

(5) Hot Tube Test

Through a glass tube having an inner diameter of 2 mm, a sample oil andair were allowed to continuously flow at rates of 0.3 ml/hr and 10ml/min, respectively, for 16 hours, while maintaining the temperature ofthe glass tube at a predetermined evaluation temperature (280 to 310°C.). Next, the color of a lacquer deposited onto the glass tube wascompared to a color specimen and was rated on the basis of standards inwhich transparency was rated as 10 points and black as 0 point. Thehigher the rating is, the better is the performance.

(6) LFW-1 Friction Test

The friction test was performed using LFW-1 friction tester as a testingmachine to measure a width of wear scar on a tested block under thefollowing conditions:

Block material: H-60Ring material: S-10Revolution speed: 1,400 rpmOil temperature: 80° C.

Load: 30 Lbs

Time: 30 minutes

(7) ISOT Test (Oxidation Stability Test)

ISOT test was performed in accordance with JIS K-2514 “Lubricating oil;Oxidation Stability Test”. Thus, an iron-copper plate was immersed in anoil sample while stirring the oil at 165.5° C. for 96 hours. Then, atotal base number was measured in accordance with JIS K-2501(hydrochloric acid method).

Preparation Example 1 Preparation of Bis(N-Octoxycarbonylmethyl)Disulfide

Octyl ester of mercaptoacetic acid was subjected to oxidative couplingaccording to the production method for the compound represented by theabove general formula (I) to obtain bis(n-octoxycarbonylmethyl)disulfide. No tri- or higher polysulfides were detected in the obtaineddisulfide compound. A sulfur content was found to be 15.8% by mass.

Namely, into a 100 ml recovery flask, 40.8 g of octyl ester ofmercaptoacetic acid and 30.8 g of dimethyl sulfoxide were placed, andthe resultant mixture was heated in an oil bath at 120° C. for 8 hours,After having been cooled, the obtained reaction mixture was dissolvedinto 100 ml of toluene and washed ten times to remove unreacted dimethylsulfoxide. Toluene was then removed by distillation under a reducedpressure to obtain 30.5 g of bis(n-octoxycarbonylmethyl) disulfide.

Preparation Example 2

Composition containing oil-soluble molybdenum Bis-succinimde wassynthesized by reacting a mixture of polybutenyl (molecular weight:1,000) succinic anhydride (PIBSA) and a polyethylene polyamine oligomer(commercially available as Polyethyleneamine E-100 from HuntsmanChemical Company) with a molar ratio of the amine to PIBSA of 0.5:1. Theproduced bis-succinimde (250 g) and 162.5 g of neutral oil were placedin a glass reactor equipped with a thermoregulator, a mechanicalagitator and a water-cooling unit. The mixture was heated to 70° C.being a reaction temperature for forming a molybdic acid salt. Then,26.6 g of molybdenum oxide and 45.8 g of water were added to the reactorwhile maintaining the reaction temperature unchanged. The temperature ofthe reactor was then maintained at 70° C. for 28 hours. After the saltforming reaction of molybdic acid had been completed, the product wasdistilled for 30 minutes at a temperature of 99° C. and a pressure of 25mmHg (absolute) or lower for about 30 minutes to remove the water. Theproduct was found to contain 4.01% by mass of molybdenum and 1.98% bymass of nitrogen.

Examples 1 to 3 and Comparative Example 1

Lubricating oil compositions having compounding formulations shown inTable 1 were prepared.

The obtained lubricating oil compositions were each measured for theirphosphorus concentrations, sulfur concentrations and sulfated ash andsubjected to the hot tube test, LFW-1 friction test and ISOT test, Theresults of the measurement and evaluation are shown in Table 1.

TABLE 1 Example Example Example Example 1 2 3 1 100N Mineral oil*¹ % bymass balance balance balance balance 500N Mineral oil*¹ % by mass 15.0015.00 15.00 15.00 Viscosity index improver*³ % by mass 6.00 6.00 6.006.00 Pour-point depressant*⁴ % by mass 0.20 0.20 0.20 0.20 ZnDTP: (as P)ppm by mass — — — 740 Disulfide*⁵ % by mass 0.13 0.06 0.06 — Ca-baseddetergent*⁶: (as Ca) ppm by mass 1100 1100 1600 1100 Hinderedphenol-based antioxidant*⁷ % by mass 0.50 0.50 0.50 0.50 Amine-basedantioxidant*⁸ % by mass 0.60 0.60 0.60 0.60 Mo-based antioxidant*⁹: (asMo) ppm by mass 100 100 100 100 Polybutenylsuccinimide*¹⁰: (as N) ppm bymass 200 200 200 200 Boron-modified polybutenylsuccinimide*¹¹: (as B)ppm by mass 400 400 400 400 Copper deactivating agent*¹² % by mass 0.100.10 0.10 0.10 Antifoaming agent*¹³ % by mass 0.10 0.10 0.10 0.10 Total100.00 100.00 100.00 100.00 P content % by mass 0.00 0.00 0.00 0.08 Scontent % by mass 0.15 0.08 0.08 0.17 Sulfated ash % by mass 0.43 0.430.59 0.59 Hot tube test — 10 10 10 8.5 280° C. merit grade Hot tube test— 8.5 9.0 10 6.0 300° C. merit grade Hot tube test — 3.0 7.5 9.0 0.0310° C. merit grade LFW-1 wear scar width mm 0.45 0.50 0.49 0.51 1400rpm, 30 Lbs, 30 min, 80° C. Base number (hydrochloric acid) after ISOTmgKOH/g 0.38 0.40 1.44 0.36 after 96 hr at 165.5° C. Remarks: *¹100NMineral oil: hydrorefined mineral oil, kinematic viscosity at 100° C. of4.5 mm²/s, sulfur content of 0.01% by mass or below *²500N Mineral oil:hydrorefined mineral oil, kinematic viscosity at 100° C. of 10.9 mm²/s,sulfur content of 0.01% by mass or below *³Viscosity index improver:polymethacrylate (weight average molecular weight of 90,000)*⁴Pour-point depressant: polyalkyl methacrylate (weight averagemolecular weight of 69,000) *⁵Disulfide: bis(n-octoxycarbonylmethyl)disulfide, sulfur content of 15.8% by mass (prepared in PreparationExample 1) *⁶Metal-based detergent: calcium sulfonate (base number of300 mgKOH/g, calcium content of 12% by mass) *⁷Hindered phenol-basedantioxidant: 4,4′-methylenebis(2,6-di-t-butylphenol) *⁸Amine-basedantioxidant: dialkyldiphenylamine (alkyl group is a mixture of butyl andoctyl groups) *⁹Molybdenum-based antioxidant: oil-solublemolybdenum-containing composition prepared in Preparation Example 2*¹⁰Ashless dispersant: polybutenylsuccinimide (nitrogen content of 0.7%by mass) *¹¹Ashless dispersant: boron-modified polybutenylsuccinimide(boron content of 0.2% by mass, nitrogen content of 2.1% by mass)*¹²Copper deactivating agent: benzotriazole *¹³Antifoaming agent:silicone oil

The results shown in Table 1 indicate as follows:

The composition of Example 1 is the same as that of Comparative Exampleexcept for using a disulfide compound according to the present inventionin place of ZnDTP in an amount corresponding to the sulfur content ofZnDTP. It is seen that heat resistance (hot tube test) of Example 1 issuperior to that of Comparative Example, while the wear resistance ofExample 1 is comparable to that of Comparative Example.

The composition of Example 2 is the same as that of Comparative Exampleexcept for using a disulfide compound according to the present inventionin place of ZnDTP in a reduced amount relative to the sulfur content ofZnDTP. It is seen that heat resistance of Example 2 is superior to thatof Comparative Example, while the wear resistance and base numbermaintaining property of Example 2 are comparable to those of ComparativeExample.

The composition of Example 3 is the same as that of Comparative Exampleexcept for using a disulfide compound according to the present inventionin place of ZnDTP in a reduced amount relative to the sulfur content ofZnDTP and for adjusting the sulfated ash to 0.6% by mass or less (byincreasing the amount of Ca-based detergent). It is seen that heatresistance and base number maintaining property of Example 3 aresuperior to those of Comparative Example, while the wear resistance ofExample 3 is comparable to that of Comparative Example.

The compositions of Examples 1 to 3 do not contain phosphorus that wouldcause catalyst poisoning. Further, as seen in Examples 2 and 3, it ispossible to reduce, without adversely affecting the wear resistance, thecontent of sulfur which would also cause catalyst poisoning. In general,there is a specified upper limit (API standard, JASO standard, etc.) forsulfated ash which represents a total content of metals which wouldcause clogging of DPF. From the standpoint of influence on DPF,therefore, the lubricating oil composition of the present invention, inwhich the disulfide compound is substituted for ZnDTP as shown inExample 3, is of significance in that it can improve the heat resistance(hot tube test) and base number maintaining property after the ISOT testwithout adversely affecting the wear resistance.

INDUSTRIAL APPLICABILITY

The lubricating oil composition of the present invention can, despiteits low ash content, low phosphorus content and low sulfur content,maintain wear resistance and is excellent in heat resistance and hasmerits that it can extend the lubricating oil change interval. Thus, thelubricating oil composition is used not only as the lubricating oil forinternal engines but also as automobile lube oils for drivinginstruments such as automatic transmissions, shock absorbers and powersteerings, and for gears; as metal working oils for metal working suchas cutting, grinding and deformation processing, and as hydraulic oils,being power transmission fluids, for transmission of power, powercontrol and shock absorbing in hydraulic systems such as hydraulicapparatuses and instruments.

1. A lubricating oil composition for an internal combustion engine,includes a base oil; (A) at least one member selected from the groupconsisting of disulfide compounds represented by the general formula(I):R¹OOC-A¹-S—S-A²-COOR²  (I) (wherein R¹ and R² each independentlyrepresent a C₁ to C₃₀ hydrocarbyl group which may contain an oxygenatom, a sulfur atom or a nitrogen atom, and A¹ and A² each independentlyrepresent a group expressed by CR³R⁴ or CR³R⁴—CR⁵R⁶ where R³ to R⁶ eachindependently represent a hydrogen atom or C₁ to C₂₀ hydrocarbyl group),and disulfide compounds represented by the general formula (II):R⁷OOC—CR⁹R¹⁰—CR¹¹(COOR⁸)—S—S—CR¹⁶(COOR¹³)—CR¹⁴R¹⁵—COOR¹²  (II) (whereinR⁷, R⁸, R¹² and R¹³ each independently represent a C₁ to C₃₀ hydrocarbylgroup which may contain an oxygen atom, a sulfur atom or a nitrogen atomand R⁹ to R¹¹ and R¹⁴ to R¹⁶ each independently represent a hydrogenatom or C₁ to C₅ hydrocarbyl group), and (B) at least one memberselected from the group consisting of alkali metal-based detergents andalkaline earth metal-based detergents, the component (B) being presentin an amount of 10 to 2,000 ppm by mass in terms of metal content. 2.The lubricating oil composition for an internal combustion engine asdefined in claim 1, wherein the component (A) being at least one memberselected from the disulfide compounds is present in an amount of 0.01 to0.50% by mass in terms of sulfur content.
 3. The lubricating oilcomposition for an internal combustion engine as defined in claim 1,wherein the alkali metal-based or alkaline earth metal-based detergentof the component (B) is a salicylate and/or a sulfonate.
 4. Thelubricating oil composition for an internal combustion engine as definedin claim 1, wherein the metal of the component (B) is Ca or Mg.